1. Field of the Invention
This invention generally relates to a facsimile system, and, in particular, to a facsimile communication control method and system.
2. Description of the Prior Art
When transmitting image information by a facsimile machine, if communication errors are produced in the image information received by a receiver facsimile machine, for example, due to poor network conditions, image information cannot be reproduced accurately at the receiver facsimile machine. In order to cope with such a situation, a facsimile machine provided with a unique error correction function devised by each facsimile machine manufacture has been proposed and image information can be transmitted to a receiver without errors by using such a facsimile machine. However, since such an error correction function is unique to a particular facsimile machine manufacture, such an error correction function is useless for communication between facsimile machines of different manufactures. In order to cope with this situation, there has been proposed a standard scheme of error correction mode in Recommendation T.30, which prescribes functions of Group 3 facsimile machines, by CCITT.
According to this recommendation, image information after compression by coding is divided into frames, each of which has a particular size, such as 256 bytes (octet; 1 byte (=1 octet)=8 bits) or 64 bytes, from the beginning, and one frame of image information is transmitted after having been arranged in the form of a particular frame FLM having the HDLC format as shown in FIG. 13a. The frame FLM includes a (front) flag sequence F comprised of a predetermined bit pattern, an address field A comprised of a predetermined bit pattern (global address), a control field C comprised of a bit pattern unique to a facsimile machine, an information field I, a frame check sequence FCS for error detection, and a (tail) flag F arranged in the order as mentioned. The information field I further includes a facsimile control field FCF, in which facsimile communication procedural signals are arranged, and a facsimile information field FIF, in which various information added to the facsimile communication procedural signals are arranged.
In this case, a facsimile communication procedural signal FCD (Facsimile Coded Data) is arranged in the facsimile control field FCF, and a frame number FNo indicating the order of frames and a frame data FDc, which is a coding of one frame size FSZ, are arranged in the facsimile information field FIF. Since the frame number FNo is comprised of 8-bit binary numbers, the number may extend continuously from "0" to "255" so that a continuous series of 256 frames is set as a block and it is so structured that the receiver requests retransmission on a block-by-block basis. If the image information of a single page of original cannot be transmitted in one block, another block is set for the remaining portion of the image information and the block is transmitted in succession.
When the receiver facsimile machine requests retransmission to the transmitter facsimile machine, the receiver facsimile machine transmits a frame of facsimile communication procedural signal PPR (Partial Page Request) to the transmitter facsimile machine, as shown in FIG. 13b. The facsimile communication procedural signal is transmitted with the inclusion of necessary parameters in a frame format similar to that of PPR. However, in the following description, it will be simply referred to as a signal PPR for the sake of simplicity. In this signal PPR, a bit pattern (PPR) indicating its identity to be a signal PPR is arranged in its facsimile control field FCF, and a 256-bit error map data EMp is arranged in the facsimile information field FIF.
In the error map data EMp, a data "0" is assigned to each of the frames having no transmission errors and a data "1" is assigned to each of the frames having transmission errors for the frame data in one block which has been transmitted in the order of the frames. Upon receipt of this signal PPR, the transmitter facsimile machine retransmits the frame data of each of these frames, to which the data "1" has been set in the error map data EMp, to the receiver facsimile machine. By carrying out this request for retransmission until all of the transmission errors have been cleared, the receiver can now record received image information without errors.
If the communication network or transmission path is poor, a request for retransmission may be issued repetitively for the same block. In such a case, it is extremely difficult to eliminate all of the transmission errors of frames under the same communication conditions. Under the circumstances, the following measure is recommended by the above-described Recommendations. That is, if a request for retransmission has been issued, for example, three times consecutively, the transmitter apprises the receiver of shifting down the transmission speed by one step by transmitting a signal CTC (Continue To Correct), and, if the receiver approves such a request for shift down by transmitting a procedural signal CTR (Response for CTC), the next transmission of data is carried out at a shifted down transmission speed. In this manner, transmission errors may be eliminated by suppressing errors by lowering the transmission speed.
However, according to such a prior art method, since any other conditions than the transmission speed remain unchanged before and after the shift down, the transmission errors are not suppressed sufficiently. Besides, since the transmission speed is lowered, there occurs another disadvantage of prolonged transmission speed.